Both internal and external power for implantable ventricular assist systems (VAS) is currently supplied by nickel-cadmium batteries which provide a rather low energy density (typically 12-14 WH/# and 1.2 WH/cu.in.). We have recently developed a proprietary chemistry for a stabilized nickel-zinc (Ni-Zn) rechargeable battery which in vented cell form has demonstrated a life of 600-800 cycles (compared to 100-150 cycles for conventional Ni-Zn cells). The vastly improved cycle life of the new stabilized chemistry vented Ni-Zn cells was verified in three independent sets of tests. In Phase I of this program, we undertook to develop a sealed version of the stabilized Ni-Zn cells and we have proven the viability of sealed Ni-Zn cell operation at twice the energy density (28-35 WH/# and 1.8-2.5 WH/cu.in.) of commercially available nickel-cadmium cells. In the Phase II program, a systematic investigation of cell components and construction is proposed as a means to develop a final cell prototype. Tasks will include refine studies of basic cell components (selection of cell geometry, zinc anode composition studies, and separator system design) as well as close monitoring of the degree to which cell overpressure is mitigated by our basic oxygen recombination approach. Design studies of both cylindrical and prismatic cells will be carried out. Optimum charging techniques will be investigated. Final design and construction of prototype batteries for a VAS will be completed and submitted for evaluative testing.